Method and apparatus for clearing an optical channel

ABSTRACT

A method and apparatus for clearing an optical channel for transmitting data through free space between a first and second location includes a light beam, wherein the light beam has a spatially and/or time-dependent modulated intensity profile, and is substantially collimated so that the intensity profile is conserved over a specified distance of operation. The light beam includes a cross-sectional profile having regions of low and high intensity, portions of which are provided for the transmission of an optical data signal. A light source wavelength and intensity are selected for types of obscurant particles having optical properties whereby the radiation pressure acts on the particles, and the particles may then be either attracted into or repelled from portions of the spatially modulated optical beam, leaving certain portions of the optical channel beam absent of obscurant particles, thereby enabling transmission of optical data through the cleared optical channel with low attenuation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation patent application claims the benefit of and priorityto U.S. patent application Ser. No. 12/036,975, filed Feb. 25, 2008, nowU.S. Pat. No. 8,139,944, issued Mar. 20, 2012, which and claims thebenefit of and priority to U.S. Provisional patent application Ser. No.60/916,708, filed May 8, 2007, which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present disclosure relates generally to free space optical signaltransmission.

BACKGROUND

Optical transmission is a key requirement in many existing systems.There is often a need for transmission of optical signals or energythrough free space. Such systems cover a wide range of applications inmany fields related to data communications, metrology, energy delivery,sensor systems, and medical instrumentation.

Key to the performance of such systems is the ability to propagate theoptical energy through the medium in which the system operates. For manysystems the medium consists of the atmosphere for a free-spacecommunication system. Several properties of the atmosphere conspire tolimit optical transmission, including atmospheric absorption, scattering(together known as extinction) and turbulence. In particular, clouds,fog, rain, dust, aerosols and smoke are especially detrimental tooptical transmission.

Certain communications technologies in development rely on ultra-low lowintensity beams, approximating single photon transmission, in which caseextinction may be especially serious. These technologies may benefitfrom methods of transmission that reduce propagation loss.

Depending on the strength of extinction and turbulence, the strength ofthe optical beam may be attenuated, and other characteristics, such asdirection and beam size, may also be adversely affected. In conventionalattempts to overcome the limiting factors, a system may be built usingmore power (i.e., more energy projection) or made larger (e.g., with alarger aperture detector to recover more of a spreading beam).

Other methods may be employed to minimize or eliminate some of theproblems. For example, in spatial diversity, multiple transmitters orreceivers may be used to enhance transmission probability. Anothersolution for free space communications consists of a so-called hybridsystem that employs a combination of both optical and either radiofrequency (RF) or microwave (MW) beams (e.g., wavelength diversity). Inwavelength diversity, an RF/MW frequency is used when necessary totransmit through a degraded optical channel. This method works becauseelectromagnetic waves are most efficiently scattered by particlesapproximately the size of the wavelength. Hence, longer wavelengths,such as RF, are not as affected by smaller particle obscurants, such asaerosol or water droplets. However, in general, RF/MW data transmissionrates are much lower due to the lower frequencies and require largerapertures to capture the more highly dispersive beams.

In any of these cases, the solution requires more equipment, complexity,weight, and size and results in higher cost of the overall system,simply because of duplication in many components.

As a result, there is a need for systems and methods for creating a lowloss optical channel for free space communications that is compact, andlow in cost and energy requirements.

SUMMARY

In accordance with various embodiments of the present disclosure,methods and systems are described that may enable clearing of obscurantsfrom an optical path and may be an enabling technology for free-spacecommunication or in adverse atmospheric conditions.

In one embodiment, a method of clearing an optical channel for opticallytransmitting data through free space between a first location and asecond location provides a clearing channel light beam of a wavelengthselected based on the optical properties of the obscurant particles andthe behavior of the obscurant particles when subjected to the radiationpressure of the light beam. The clearing channel light beam is spatiallymodulated to have a given intensity profile, wherein the profileincludes regions of low and high intensity. The clearing channel lightbeam is collimated such that the intensity profile is substantiallyconserved over a specified distance range of operation. The light beamis projected from a first location to a second location. The clearingchannel light beam shape is such that obscurant particles are attractedinto the high intensity portions of the spatially modulated light beam,leaving low intensity portions of the clearing channel light beam clearof the obscurant particles.

In accordance with another embodiment, the clearing channel light beammay be formed in a shape described mathematically as including Bessel,Laguerre-Gaussian, or equivalents, where the clearing channel light beamhas profile regions of low and high intensity.

In accordance with another embodiment, an optical data channel isprovided within the minimum intensity portion of the center of theclearing channel light beam. Transmission of the optical data channelmay take place from the first location to the second location or fromthe second location to the first location, where the clearing channellight beam is provided by one of the first or second locations.

In accordance with another embodiment, the spatially modulated clearingchannel light beam profile may be temporally modulated in the radialdirection, wherein the spatial pattern of the light beam expands fromthe center outwardly, removing the obscurant particles from the centerof the light beam, maintaining a clear channel for the optical datasignal beam.

In accordance with another embodiment, the spatially modulated lightbeam may be time independent, wherein the spatial pattern of theclearing channel light beam removes the obscurant particles from thecenter of the light beam continuously, maintaining a clear channel forthe optical data signal beam.

In accordance with another embodiment, a method of clearing an opticalchannel for optical transmission of data provides a clearing channellight beam which has a time independent spatially modulated intensityprofile. The clearing channel light beam is collimated so that theintensity profile is substantially conserved over a specified distancerange of operation. The profile of the clearing channel light beamincludes regions of low and high intensity in a cross-sectional area,portions of which are provided for the transmission of an optical datasignal. The clearing channel light beam has a wavelength selected suchthat the obscurant particles have optical properties whereby theradiation pressure acts on the obscurants such that they are repelledfrom the high intensity portion of the spatially modulated optical beam,leaving the region absent of obscurant particles and thus clear fortransmission of optical data. The clearing channel light beam isgenerated at one location and directed to a second location. An opticaldata channel is introduced to propagate within the cleared channelbetween the two locations. Transmission of the optical data signal beammay take place in either direction.

In accordance with another embodiment, a method of clearing an opticalchannel for optical transmission of data provides a clearing channellight beam which has a spatially modulated intensity profile which isalso time-dependent spatially modulated. The clearing channel light beamis collimated so that the intensity profile and its dimensions aresubstantially conserved over a specified distance range of operation.The profile of the light beam includes regions of low and high intensityin a cross-sectional area portions of which are provided for thetransmission of an optical data signal. The clearing channel light beamhas a wavelength selected such that the obscurant particles have opticalproperties whereby the radiation pressure acts on the obscurants suchthat they are attracted into the high intensity portion of the spatiallymodulated clearing channel light beam, leaving the region absent ofobscurant particles and thus clear for transmission of optical data. Thetime-dependant spatially modulated beam is adapted to sweep obscurantparticles from a region of the optical beam on a periodic and/orcontinuous basis related to the time-dependent modulation. The clearingchannel light beam is generated at one location and directed to a secondlocation. An optical data signal beam is introduced to propagate withinthe cleared channel between the two locations. Transmission of theoptical data signal beam may take place in either direction.

In accordance with another embodiment, a method of clearing an opticalchannel for optical transmission of data provides a clearing channellight beam which has a spatially modulated time-independent intensityprofile. The light beam is collimated so that the intensity profile issubstantially conserved over a specified distance range of operation.The profile of the clearing channel light beam includes regions of lowand high intensity in a cross-sectional area portions of which areprovided for the transmission of an optical data signal. The light beamhas a wavelength selected such that the obscurant particles have opticalproperties whereby the gradient force and radiation pressure act on theobscurants such that they are repelled from the high intensity portionof the spatially modulated optical beam, leaving the region absent ofobscurant particles and thus clear for transmission of optical data. Thetime-dependant spatially modulated beam is adapted to actively sweepobscurant particles from a region of the optical beam continuously on aperiodic and/or continuous basis related to the time-dependentmodulation. The clearing channel is generated at one location anddirected to a second location. An optical data signal beam is introducedto propagate within the cleared channel between the two locations.Transmission of the optical data signal beam may take place in eitherdirection.

In accordance with another embodiment, a method of clearing an opticalchannel for optical transmission of data provides a clearing channellight beam to clear an optical channel of obscurant particles, where theparticles removed include water droplets in clouds and fog, dust, smoke,and aerosols of various types.

In accordance with another embodiment, a system and apparatus forclearing an optical channel for optical transmission of data includes aclearing channel light beam, a beam-forming optical element for shapingthe beam spatially and statically in time, a beam collimating elementfor shaping the beam in a collimated shape for maintaining asubstantially fixed dimensional profile over a specified distance range,and an optical element for inserting an optical data signal beam forpropagation of optically transmitted information within the clearingchannel light beam. The optical data signal generating apparatus may beco-located with the first location channel clearing apparatus, or it maybe at the second location, and the optical data signal beam may betransmitted in either direction.

In accordance with another embodiment, a system and apparatus forclearing an optical channel for optical transmission of data, where achannel clearing light beam and an optical data signal beam havesubstantially the same wavelengths, includes a beam splitter forseparating portions of a light beam into the channel clearing light beamand the optical data signal beam, a beam modulator for time divisionseparation of the two beams temporally, a data signal modulator forimposing data modulation on the optical data signal beam, and a beamcombiner/splitter for recombining the two beams.

In accordance with another embodiment, a system and apparatus forclearing an optical channel and optical transmission of data includes afirst light source, a second light source providing a second light beam,wherein the wavelengths of the two light sources are substantiallydifferent, a beam-forming optical element for shaping a channel clearinglight beam from the first light source both spatially and dynamically intime, a beam forming element for shaping the beam in a collimated shapefor maintaining a substantially fixed dimensional profile over anextended range, a signal modulator means for imposing data on the secondlight beam light source to provide an optical data signal beam, and anoptical element for inserting the optical data signal beam forpropagation of optically transmitted information within the clearedchannel light beam.

In accordance with another embodiment, a system and apparatus forreceiving data transmitted over an optical data signal beam through aclearing channel light beam, wherein the clearing channel light beam andthe optical data signal beam have substantially different wavelengths,includes an optical detector, a spatial filter, a wavelengthdiscriminating filter and/or beam splitter for separating the clearingchannel from the data channel, a beam-forming optical element forcollecting the transmitted data channel, and a detector for receivingand processing the data.

In accordance with another embodiment, a method for clearing obscurantsfrom and communicating data through a communication channel comprisesselecting a wavelength for a channel clearing beam of light based on adesired behavior of the obscurants when subjected to optical trappingforces exerted on the particles when irradiated with the channelclearing beam. The channel clearing beam is transmitted through thechannel so as to clear the obscurants from selected portions thereof Forexample, a data signal beam of light is also transmitted through thechannel, and one or more of the polarization, shape, intensity and timeof occurrence of one or both of the channel clearing beam and the datasignal beam are modulated in such a way that respective portions of thetwo beams having one or more of substantially the same polarizations,shapes, intensities and times of occurrence do not occupy the sameportion of the channel at the same time. As another example, one or moreof the wavelength, polarization, shape, intensity and/or time ofoccurrence of one or both of the channel clearing beam and the datasignal beam are modulated such that the obscurants are substantiallyunaffected by the data beam, and the channel clearing beam and the databeam are mutually incoherent and/or do not occupy the same portion ofthe channel at the same time.

The scope of the disclosure is defined by the claims, which areincorporated into this section by reference. A more completeunderstanding of embodiments of the present disclosure will be affordedto those skilled in the art, as well as a realization of additionaladvantages thereof, by a consideration of the following detaileddescription of one or more embodiments. Reference will be made to theappended sheets of drawings that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

FIGS. 1A and 1B illustrate the concept of optical trapping.

FIG. 2 illustrates examples of optical beams with on-axis minima.

FIG. 3 illustrates the property of beam invariance for propagation overextended distances.

FIG. 4 illustrates the effect of self-healing in certain types of beams,such as Bessel light modes and other modes with conical wave vectors.

FIG. 5 illustrates a process of forming a free space optical channel inaccordance with an embodiment of the present disclosure.

FIG. 6 illustrates a process of shaping a channel clearing beamspatially and temporally, in accordance with an embodiment of thepresent disclosure.

FIG. 7 is a block diagram indicating components of an apparatus forgenerating a channel clearing beam and transmitting a data signal inaccordance with an embodiment of the present disclosure.

FIG. 8 is a block diagram indicating components of an apparatus forgenerating a channel clearing beam and transmitting a data signal inaccordance with another embodiment of the present disclosure.

FIG. 9 is a block diagram indicating components of an apparatus forreceiving a data signal transmitted through a channel clearing beam, inaccordance with the embodiment shown in FIG. 7.

FIG. 10 is a block diagram indicating components of an apparatus forreceiving a data signal transmitted through a channel clearing beam, inaccordance with the embodiment shown in FIG. 8.

DETAILED DESCRIPTION

In free space communication, the atmosphere severely limits the abilityto transmit optical wavelength photons because of obscuration fromaerosols and particles in the air.

As described herein, various embodiments in accordance with the presentdisclosure may utilize novel adaptations of optical trapping and beamwave front shaping to create a low-loss channel for opticaltransmission.

Its use may cover a wide range of applications such as, for example,between communication points within a building, between buildings, andbetween ground station sites and airborne sites, such as a manned or anunmanned aerial vehicle (UAV). Data security may be strongly enhancedbecause the transmission path is narrowly directed and not easilyintercepted without also providing an indication that interference isoccurring.

Related techniques as will be described have been used for the pastdecade in optical “tweezers” experiments and methods to manipulate andmove microscopic particles, including biological and molecularstructures and are adapted herein to provide novel use. The methods andsystems disclosed herein rely on the combination of the basic physics ofthe phenomenon known as optical trapping and various unique propertiesof light, such as propagation invariance, self-healing, and on-axisminima associated with certain types of optically shaped beams toprovide novel applications in accordance with the present disclosure.The unique combination of optical trapping and beam shaping is availableusing technologies such as lasers, optics, holograms, spatial lightmodulators (SLMs) and acoustic-optic deflectors (AODs) to manipulate,move and clear obscurants from an optical path (channel), thus reducingoptical attenuation.

The conditions for optical control of atmospheric obscurants depend on anumber of factors. Mean atmospheric conditions (e.g., temperature andpressure, density of obscurants) may be important. The obscurants foundin the atmosphere may possess a range of optical refractive index,transparency and absorption that vary with optical wavelength. Shape andmass are also relevant parameters. These may determine whether aparticle is drawn into the high intensity portion of a beam or repulsedfrom it, and the ratio of optical pressure (i.e., resulting force) toparticulate mass will determine the acceleration, terminal velocity andrate of motion in response to optical radiation pressure and the opticalgradient force.

Systems and methods are disclosed herein to provide means to clear anoptical channel by using novel optical beams. Such optical beams may beshaped to enhance the optical pressure that may be produced to controlthe accumulation or displacement of particles in the path defined by theoptical channel. A non-uniform beam with sufficient gradients inintensity will create optical pressures capable of moving particles. Forexample, an optical beam may be formed to have a Laguerre-Gaussian orBessel profile with sufficient intensity that it may sweep lightscattering obscurant particles, such as water droplets and aerosols, outof the optical channel path. Various Bessel orders of such beams, forexample, may have either a maximum or a minimum of optical intensity atthe on-axis center of the beam. For obscurant particles that areattracted to a high intensity portion of a beam, it may be advantageousto form a beam with an intensity minimum at the center, and to attractsuch particles away from the on-axis center, thus creating a “pipe” forlow-loss light transmission. In other cases, where the optical nature ofthe obscurant particles may be such that they are repelled from highintensity regions of a light beam, it may be advantageous to form achannel clearing beam with a maximum at the center. Other beam shapes,classified according to a variety of mathematical definitions, may beconsidered equivalent and also be employed, and may generally haveeither of the two intensity profile features mentioned, i.e., a centralmaximum or minimum. Dependence of the responding behavior of obscurantson their optical properties as a function of wavelength determines thepreferred profile for the clearing channel beam.

By periodically clearing the optical spatial channel, transmitting thedesired information, and re-clearing the channel as necessary, thistechnique may enable optical signals that otherwise would be quicklyattenuated to be transmitted over large distances. The optical clearingbeam may be static or dynamic. A static beam would continuously clearthe channel. Alternatively, it could be periodically pulsed. A dynamicbeam may change its shape and/or intensity, in order to be effective insweeping a broad range of obscurant particle types out of the channel.

To be effective for a particular application, the beam profile andintensity must be maintained within specified operational limits. Thus,for example, where free space communication is required between twobuildings, the operational range/distance may be on the order of severaltens to hundreds of meters. Longer range applications may have morestringent requirements. The clearing channel would be required to staywithin a certain beam width or diameter over the specified distance tosatisfy energy density requirements or receiver aperture limits.

A conventional means for specifying the size of a beam with respect to aGaussian profile is to state the diameter at which the beam powerdensity falls to (1/e)² of the peak value (nominally at Gaussian beamcenter). Since the channel clearing beams described in accordance withembodiments of the present disclosure need not be Gaussian and may havea power density null at the center, it may be convenient to define anominal beam waist diameter differently. The definition to use may bethe diameter, measured radially, at which the power density of the firstlobe (i.e., first ring) of the channel clearing beam falls to (1/e)² ofthe peak value of the lobe, and require that the peak value of the lobeequal or exceed a minimum power density over the operational distance.Another requirement may be that the (1/e)² beam waist size spreadingdoes not exceed a specified value over the operational distance. Asimilar requirement for intensity profile may be specified for thecentral portion of a beam having a central minimum, as it may bepreferred to have a satisfactory gradient in the channel clearing beamintensity, measured from the beam center to the maximum of the firstlobe, to effectively “sweep” obscurant particles away from the center,creating the data communication channel “pipe.”

The fundamental concept of optical trapping is illustrated in FIGS. 1Aand 1B. In FIG. 1A, a light beam with a Gaussian intensity distributionis focused on a spherical “bead.” The bead may be representative of awater vapor droplet, aerosols, dust or other atmospheric particles thatare mainly responsible for scattering light. In this example, theoptical beam is refracted, and thus deflected to the left, which resultsin development of a force that pushes the bead to the right due to thenet dipole force. The bead will eventually come to rest directly in linewith the intensity maximum at the center of the beam, where netdeflection forces laterally (i.e., x and y) are balanced. Thus, the beadbecomes “trapped” at the maximum of the Gaussian beam.

In FIG. 1B, a bead is vertically trapped due to the light force asfollows: Consider two Gaussian beams configured to converge initially ata point in front of the bead. The bead will be drawn to the maximumintensity point(as explained for x-y trapping) at the intersection ofboth beams, which is generally upward, but also has lateral componentspertaining to each beam, since each beam is directed at an angle towardthe particle. The two horizontal components of force may cancel, leavinga net upward vertical force. The bead experiences a net vertical forceupward until the point of convergence of the two beams reaches a pointsubstantially near the center of the bead, at which point the netvertical forces of the incident and exiting beams may cancel. In thiscircumstance, the bead is “trapped” in the z-direction as well as thex-y direction. A single beam that converges to a focal point willsimilarly trap the bead at the focal point, i.e., the point of highestoptical power density.

While conventional optical trapping generally uses highly focusedGaussian beams, adapting the shapes of such light beams may be moreuseful for control of particles that affect light transmission. Beamsshaped to have one or more intensity minima and one or more highintensity maximum may be configured to displace particles from a regionof free space through which optical transmission is sought. For example,FIG. 2 illustrates examples of Laguerre-Gaussian (LG) profile beams,where intensity profiles may have on-axis minima and consist of a seriesof concentric rings. Beams shaped in this manner may be used to trapparticles in the high intensity portion of the ring(s) and removeparticles from the central minimum portion. LG and Bessel beams are twoexamples of beam types that exhibit such shape properties and that maybe adapted for this purpose.

Another useful property is exhibited by beams such as, or similar to,Bessel beams—that of beam propagation invariance. This is illustrated inFIG. 3 where the beam maximum substantially maintains its shape duringpropagation for a longer distance than is generally observed fromdiffraction of plane optical waves. This property arises because thebeam is constructed from a non-planar ensemble of waves with an alteredpropagation wave front. That is, the waves may move on a conicalsurface. This property may be useful for beam trapping throughout a longchannel path.

Some beams that exhibit propagation invariance also have another uniqueand useful property—that of self-healing. This is illustrated in FIG. 4.With these beams, although a portion of the beam is blocked by anobscurant particle, as it propagates past the particle, the beamself-heals and restores itself to substantially its originalconfiguration. The advantage of this property is that the beam continuesto perform the channel clearing function even as it propagates through ascattering medium in the atmosphere.

One embodiment of the concept may be to use a static beam, such asillustrated in FIG. 2. The beam shape may be spatially static, but thebeam itself could be pulsed on and off periodically to clear thechannel, since particles that drift into the higher intensity regionsmay be trapped and swept from the center of the channel. Then, once thechannel is cleared, a signal beam (at the same or a differentwavelength) may be transmitted down the cleared channel. When theclearing beam is turned off, particles will drift back into the channel.When the transmission becomes critically attenuated below a levelspecified for an application, the clearing beam may be used again. Thisis an example of time division multiplexing of the channel clearing beamand the data channel beam.

Specialized beams, such as LG and Bessel beams (and others exhibitingsimilar features), may be created with spatial light modulators (SLMs)that act as real-time holograms. One vendor of such devices is BoulderNonlinear Systems (Lafayette, Colo.). Other devices that may be used todynamically configure beams in this manner are acousto-optic deflectors(AODs) and acousto-optic modulators (AOMs). Newer SLMs may be programmedto change their configuration rapidly and hence modify the beamconfiguration rapidly. Thus, one could imagine a donut shaped beam whoseradius starts small and gradually increases. Such a beam may be used tosweep particles radially out of the center of the channel.

FIG. 5 is a process flow illustrating a method 500 of forming a freespace optical channel in accordance with an embodiment of the presentdisclosure. A light source wavelength λ₁ is selected (block 510) wherebyobscurant particles will react to the light radiation pressure in aknown way based on the optical properties of the obscurants. A lightsource may include a laser or another light source. In the case of alaser, however, the coherent nature of the laser beam results in ahigher optical power density than with incoherent light, and maytherefore be advantageous. A light beam of the selected wavelength (orwavelengths) is provided by a light source (block 520) at a firstlocation (not shown). The light beam is shaped (block 530) by anappropriate optical element (not shown), such as a hologram, axicon,customized lens, SLM, AOM or an equivalent device that appropriatelyalters the beam wave front, providing a shaped beam. The shaped beam isthen substantially collimated (block 540) to provide a channel clearingbeam 550 for propagation over a distance suitable for the applicationand to minimize loss of optical power efficiency in the clearing channeldue to lateral spatial dispersion as the beam propagates.

A beam combining element, such as, for example, a beamsplitter/combiner, is placed in the beam (block 560) to enableintroduction of a data channel signal 745 (as described with referenceto FIG. 7, below), positioned to propagate through the channel formed bythe channel clearing beam 550. The beam splitter/combiner, thepolarization of the channel clearing beam 550 and the polarization ofthe data channel signal 745 may be configured such that channel clearingbeam 550 and data channel signal are combined to propagate withorthogonal polarizations. The data channel signal is presumed to be awell collimated beam as well, subject to the same beam spatialdispersion constraints as the channel clearing beam 550. The combineddata channel signal and channel clearing beam 550 may be furtherdirected (block 570) by a beam projector toward a second location (notshown), where it is received.

The intensity of the data channel signal 745 may be specified to be lessthan a maximum intensity such that substantially no particle clearingfrom the optical channel is caused by the data channel. Therefore, onlythe channel clearing beam may provide the channel clearing function.

FIG. 6 illustrates one embodiment of shaping a channel clearing beamspatially and temporally, which may be used in block 530 of FIG. 5. Beamshaping (block 530) may include two distinct processes, namely, spatialshape modulation (block 534) and temporal/intensity modulation (block536), which may be performed in parallel, serially, or individually tothe exclusion of one or the other of blocks 534 and 536. Spatial shapemodulation (block 534) may be accomplished with the use of an SLM (notshown) or similar devices (described below) to reshape the wave front ofthe light beam, which is typically provided initially as a plane wave.

The resulting beam may take various forms in detail, such as a Bessel orLaguerre-Gaussian beam, or other beams of higher order (not shown), thathave the property of being a non-planar wave wherein the central portionof the beam may have either an optical null (minimum) or a maximum, asdesired, with alternating surrounding regions of minimum and maximumoptical intensity. These beams have various advantages, such aspropagation invariance and self healing, as described above.

The light beam may be further modulated in time (i.e., temporally) (inblock 536) to variously turn channel beam 550 on or off, vary theintensity, or to vary the shape over time. These functions may have theadvantage of clearing a variety of obscurant particles of varying typesand properties.

FIG. 7 is a block diagram indicating the components of an apparatus 700for generating a channel clearing beam and transmitting a data signal inaccordance with an embodiment of the present disclosure. A light source710 provides a beam of suitable wavelength λ₁ and power, selectedaccording to the optical properties of the atmospheric obscurantparticles. More than one light wavelength may also be provided, forexample, by using more than one light source, or by operating a lightsource in multi-wavelength mode.

Beam shaping optics 720 receives the light beam and alter the wave frontto provide a beam suitable for channel clearing. Optics that may be usedfor this purpose include axicons and other specialized lenses, SLMs,AOMs, AODs, holograms, or equivalent structures that alter the phase ofthe light wave front to produce a beam with a modulated intensityprofile, and may additionally have the properties of propagationinvariance and self-healing. For example, Bessel beams (not shown) arecircular, and may have a minimum or maximum at the center of the axis ofpropagation, and may have one or more rings of maximum and minimumintensity. Laguerre-Gaussian (LG) beams (not shown) have substantiallythe same characteristics, and may differ only in detail.

For example, the beam formed, i.e., channel clearing beam 550, may havethe shape of one or more rings, as shown in FIG. 2, with an optical null(minimum) in the center. The resulting beam is then substantiallycollimated and sized with appropriate collimator/sizing optics 730, suchas lenses, focusing mirrors, or the equivalent, to provide a beam ofsuitable diameter. In accordance with one embodiment of the presentdisclosure, where the channel clearing beam 550 is formed with a minimumat the beam center, a beam splitter/combiner 740 may be placed in thepath of the channel clearing beam 550 to permit a data channel signalbeam 745 that may be of a different wavelength λ₂ provided from a datamodulated light source 742 to be introduced at the center of the channelclearing beam 550 and directed to propagate collinearly with the channelclearing beam 550. The combined beam may then be further directed with aprojection optics 750 from the first location (not shown) of theapparatus 700 to the second location (not shown), where the data channelsignal beam 745 and channel clearing beam 550 are received andseparated.

FIG. 8 is a block diagram indicating the components of an apparatus 800for generating a channel clearing beam and transmitting a data signal inaccordance with a second embodiment of the present disclosure. The lightsource 710, beam shaper 720, collimator/sizing optics 730, beamsplitter/combiner 740, and projection optics 750 function substantiallyin the same manner as in the apparatus 700. In this case, however, asplitter 815 extracts a fraction of the beam from the light source 710.The extracted portion is subjected to spatial filtering, beam controland preparation by a group defined collectively as signal optics 825 fortransmission through a signal modulator 835 that imposes data on theextracted beam fraction.

A clock switch 855 operates to alternately turn on the beam shaper 720and impose a clocking pulse on the shaped beam, and trigger a datasignal generator 865 to operate a signal modulator 835. Thus, by timedivision multiplexing, one light source 710 is used to provide a lightsource for the clearing channel beam 550 and the data signal channelbeam 745. In this case, the two beams may not be operationalsimultaneously. Further components defined by the signal optics 835direct a modulated data channel signal beam 745 to the beamsplitter/combiner 740, where it is introduced collinearly with thechannel clearing beam 550. The combined beams are then directed via theprojection optics 750 from the first location of the apparatus 800 tothe second location (not shown), where the data channel signal beam andchannel clearing beam are received and separated.

FIG. 9 is a block diagram indicating the components of an apparatus 900for receiving a data channel signal beam 745 transmitted through achannel clearing beam 550, in accordance with the embodiment of thepresent disclosure shown in FIG. 7. A spatial filter 910 may optionallybe provided to remove all or a substantial portion of the clearingchannel beam 550 of wavelength This is advantageous in reducing therelatively higher energy content of the channel clearing beam 550 thatwould otherwise reach the detector, adversely masking the content of thesmaller amplitude data channel signal beam 745.

A wavelength dependent filter 920 is selected and placed tosubstantially block all of wavelength λ₁, further reducing or removingremnants of channel clearing beam 550. The wavelength dependent filter920 may include a thin film optical filter, a prism beam splitter, orequivalent optical elements with specific characteristics fortransmitting the wavelength (or range including the wavelength) λ₂,while reflecting or otherwise blocking the wavelength (or range ofwavelengths) specified by λ₁. The wavelength dependent filter 920 mayinclude a plurality of such elements disposed at one or more positionswithin the receiving apparatus 900, and is only indicated once forconvenience. A set of one or more focusing optics 930 provide efficientcollection of the data channel signal beam 745 on a data channeldetector 940.

FIG. 10 is a block diagram indicating components of a receivingapparatus 1000 for receiving at a second location data channel signalbeam 745 transmitted through channel clearing beam 550, in accordancewith the embodiment of the present disclosure shown in FIG. 8. Forexample, the clearing channel beam 550 and the data channel signal beam745 may be prepared from the same light source, or from a second lightsource, but at substantially the same wavelength λ₁.

As described above, the channel clearing beam 550 and data channelsignal beam 745 may be time division multiplexed, and therefore, are notoperational simultaneously. A portion of the combined composite beam maybe “picked off” with a partial beam splitter 1010. The beam splitter1010 may be, for example, a neutral density filter of low density toacquire a small portion of the entire beam energy. A beam detector 1020converts the optical signal received to an electrical signal, which isthen processed by a timing circuit 1030 enabled with logic configured toidentify a timing pulse that indicates the presence or start of theclearing channel beam 550 as a pulse lasting for a specified timeinterval. Additional pulses and logic may be included to detect the endof the pulse time interval of the clearing channel beam 550.

By comparison, the small fraction of the data channel signal beam 745that is also received at the beam detector 1020 may be low enough thatit is below a detection/trigger threshold, thus indicating the timeinterval during which data is being received. The logic circuitry of thetiming circuit 1030 may be used to operate an optical shutter 1040 toclose when the channel clearing beam 550 is operational, and open whenthe data channel signal beam 745 is operational. The optical shutter1040 may include an electro-optic shutter, a liquid crystal shutter,SLMs, AOMs, or mechanical shutters, provided the requirements forresponse speed to pass the data channel signal beam 745 and block thechannel clearing beam 550 may be satisfied.

Numerous advantages may be realized with systems and methods asdescribed above, as well as additional combinations and embodiments ofthe present disclosure. For example, the reduction of atmosphericattenuation of light using the methods and systems described herein mayenable a direct line-of-sight transmission of data between two locationsat very high data rates commonly employed in fiber optictransmission—e.g., tens to hundreds of gigabits per second or more.Deployment of such systems and methods may eliminate the need for costlyinstallation of land-line cables.

Furthermore, the channel clearing beam has been described as beingco-located at the first location with the source of data channel signalbeam and directed toward the second location. However, data may be sentfrom the second location through the channel clearing beam provided bythe first location. Thus, configurations of communication systems may becontemplated (such as “star” networks) where a channel clearingapparatus is not required at all locations, resulting in possiblysignificant cost savings.

Another benefit may be realized using free space optical transmissionfor energy distribution via optical power, particularly for specializedapplications. For example, electrically driven unmanned aerial vehicles(UAVs) may receive power by transmission from a first location groundlocation that generates a dynamically projected channel clearing beamthrough which it supplies optical energy received by solar-panel-typedetectors configured on the UAV. Reciprocally, the UAV, as the secondlocation, may transmit data back to the ground station. It may beappreciated that the UAV need not be required to carry fuel or anapparatus for generating the channel clearing beam, and thus may be moreefficiently dedicated to its mission.

Embodiments described above illustrate but do not limit the disclosure.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the presentdisclosure. Accordingly, the scope of the invention is defined only bythe following claims.

What is claimed is:
 1. A method for clearing obscurants from andcommunicating data through a communication channel, the methodcomprising: selecting a wavelength for a channel clearing beam of lightbased on a desired behavior of the obscurants when subjected to opticaltrapping forces exerted on the particles when irradiated by the channelclearing beam; transmitting the channel clearing beam through thechannel so as to clear the obscurants from selected portions thereof;transmitting a data signal beam of light through the channel; and,modulating one or more of the wavelength, polarization, shape, intensityand/or time of occurrence of one or both of the channel clearing beamand the data signal beam such that the obscurants are substantiallyunaffected by the data beam, and the channel clearing beam and the databeam are mutually incoherent and/or do not occupy the same portion ofthe channel at the same time.
 2. The method of claim 1, furthercomprising collimating the channel clearing beam.
 3. The method of claim1, wherein the transmitting of the data signal beam comprises combiningthe data signal beam with the channel clearing beam.
 4. The method ofclaim 1, wherein the channel clearing beam and the data signal beam havethe same wavelengths.
 5. The method of claim 1, wherein the channelclearing beam and the data signal beam are transmitted through thechannel in the same direction.
 6. The method of claim 1, furthercomprising shaping one or both of the channel clearing beam and the datasignal beam such that one or both of the beams respectively exhibits oneor both of propagation invariance and self healing.
 7. The method ofclaim 3, wherein the modulating comprises polarizing one or both of thechannel clearing beam and the data signal beam such that theirrespective polarizations are substantially orthogonal to each other. 8.The method of claim 1, wherein the modulating comprises pulsing thechannel clearing beam and the data signal beam on an off such thatrespective pulses of the two beams produced thereby are substantially180 degrees out of phase with each other.
 9. The method of claim 1,wherein the channel clearing beam and the data signal beam are producedby the same light source.
 10. The method of claim 1, wherein one or bothof the channel clearing beam and the data signal beam comprises coherentlight.
 11. An apparatus for clearing obscurants from and communicatingdata through a communication channel, the apparatus comprising: a firstlight source configured to produce a channel clearing beam of light at awavelength operable to effect a desired behavior of the obscurants whensubjected to optical trapping forces exerted on the particles whenirradiated by the channel clearing beam; a first transmitter fortransmitting the channel clearing beam through the channel so as toclear the obscurants from selected portions thereof; a second lightsource configured to produce a data signal beam of light; a secondtransmitter for transmitting the data signal beam through the channel;and, a modulator for modulating one or more of the wavelength,polarization, shape, intensity and/or time of occurrence of one or bothof the channel clearing beam and the data signal beam such that theobscurants are substantially unaffected by the data beam; and, thechannel clearing beam and the data beam are mutually incoherent and/ordo not occupy the same portion of the channel at the same time.
 12. Theapparatus of claim 11, further comprising a collimator for collimatingthe channel clearing beam.
 13. The apparatus of claim 12, wherein one orboth of the first and second light sources comprises a laser.
 14. Theapparatus of claim 11, wherein the modulator comprises a channelclearing beam shaper and a data signal beam modulator.
 15. The apparatusof claim 14, wherein the channel clearing beam shaper comprises a lens,a spatial light modulator (SLM), an acousto-optic deflector (AOD), or anacousto-optic modulator (AOM).
 16. The apparatus of claim 15, whereinthe lens comprises an axicon.
 17. The apparatus of claim 14, wherein thesecond transmitter comprises a beam combiner.
 18. The apparatus of claim17, wherein the modulator further comprises a beam splitter.
 19. Theapparatus of claim 18, wherein the modulator further comprises a clockswitch.
 20. The apparatus of claim 17, further comprising a projectorfor projecting a combination of the channel clearing beam and the datasignal beam.